document tree: [i0 -> [x-foo]]
x-foo's shadow tree: [j5 -> [x-bar] -> j6]
x-bar's shadow tree: [k1 -> k0 -> [s2]]
slot #s2: [j1 -> j2 -> j3 -> j4 -> [s1] -> j0]

slot #s1: [i1 -> i2]
v1 ideal nav forward: [i0 -> j5 -> xbar -> k1 -> k0 -> j1 -> j2 -> j3 -> j4 -> i1 -> i2 -> j0 -> j6]